The present invention relates to a device which senses the degree of angular rotation of a shaft, and particularly to a device providing an output voltage linearly proportional to the degree of rotation.
Devices for detecting rotary speed of a toothed element such as a gear are well known. See, e.g., U.S. Pat. No. 4,481,469. Such a device utilizes a Hall cell sensor or a magneto-resistive (MR) sensor and a permanent magnet to generate a sinusoidal voltage based on the tengential component of the magnetic field as teeth pass the sensor. Each passing soft iron gear tooth causes a peak in the sinusoid which can readily be transparent to RPM; such a device is thus a digital speed measuring device. U.S. Pat. No. 4,086,533 discloses apparatus which utilizes a Hall cell sensor to determine the angular position of a rotating part, but only identifies discrete positions where a magnetic field component between two magnets shifts direction under the influence of rotating flux guides.
For some applications it is desirable to know the exact angular position of a shaft as it moves from a first angular position to a second angular position rather than measuring speed or discrete locations of the shaft. For example, a pedal position sensor in an automobile could determine throttle position from pedal shaft position without direct mechanical linkage. The relationship between the angular position of such a shaft and the output voltage of the sensing device should be linear.
One prior art solution to providing such a linear voltage output is to fix a magnet on the end of the shaft, as shown in FIG. 1A. Pedal shaft 10 is pivoted by pedal 12 against the action of a spring on the opposed lever 13, and thus rotates in bearings 14. The magnet 17 is mounted on a coaxial shaft extension 16 with its magnetic axis perpendicular to the shaft axis and parallel to the sensing substrate 20 so that flux lines pass substantially perpendicular to the plane of the substrate and the sensor thereon. The substrate 20 in turn is mounted to printed circuit board (PCB) 23 by leads 22. The substrate carries an MR sensing element comprising permalloy strips whose resistance changes in the presence of a changing external magnetic field therethrough. This could be described in rectangular coordinates by field components H.sub.x and H.sub.y. In the initial pedal position H.sub.x could be zero, and would increase to maximum value when shaft 10 rotated ninety degrees, whereupon H.sub.y would be zero. FIG. 1B shows a similar arrangement using two magnets 25 on a frame 26. This arrangement and other uses of MR sensors is described in greater detail in Philips Data Handbook, Book S-13, pp. 4-19 (1986), which pages are hereby incorporated by reference.
FIG. 1A also depicts another prior art sensing arrangement wherein two magnets 18 are mounted directly on the shaft 10, with a sensing substrate 20 in a plane perpendicular to the axis of shaft 10. Such an arrangement will also provide a continuous voltage variation as the shaft rotates, which voltage variation will be substantially linear for rotations less than the angular separation of magnets 18.
The above arrangements require mounting magnets on the shaft whose rotation is to be measured, which from a manufacturing standpoint is not as expedient as mounting an integrated sensing device adjacent the shaft.